For a brief period between November 12 and 20 Turkish jets did not enter Syrian airspace to support its FSA proxy. This was because the Syrian regime activated its air defense systems and threatened to shoot down any Turkish jets following Turkey’s bombing of SDF positions on October 19 – which Ankara carried out to prevent a Kurdish-led advance from the city of Manbij (which was captured from ISIS in August) to al-Bab.

Both, the Syrian and Turkish militaries have been considerable weakened by different internal events. Since 2011 the Syrian Army’s manpower has been gradually sapped by this costly war of attrition and desertions. At present, al-Assad’s army is so overstretched it relies heavily on support from militias. The Turkish Army, on the other hand, has faced large-scale purges following the July 15 coup attempt, with tens-of-thousands of personnel being detained and a reignited Kurdish insurgency in Turkey’s eternally volatile southeast. The question of whether it remains even functional has been raised in the Turkish press. The Turkish Air Forces’ ability to field large numbers of its jet fighters is also in question given the large number of pilots who have been discharged.

Aside from activating its air defenses and threatening to shoot down Turkish jets, Syria has demonstrated that it is willing to forcibly resist a Turkish advance on al-Bab. A Syrian helicopter dropped barrel bombs on Turkey’s FSA proxies in the village of Tal Nayif, south-east of Dabiq, on October 26, killing two of the FSA-rebels and wounding five. More alarmingly, the Turkish Army announced on November 24 that a Syrian airstrike killed three of their soldiers in Syria. How far both sides are willing to, and can, escalate clashes is unclear given the reluctance of their powerful patrons to support them and the aforementioned state of their armed forces.

Beginning on August 24, operation “Euphrates Shield” has been relatively ramshackle, relying on an estimated 1,500-3,000 relatively basically trained irregular FSA fighters to advance and rout ISIS with heavy fire support. Some of these fighters have already been killed in deadly ISIS ambushes and by ISIS’s notoriously lethal booby traps. And this has only been in fighting the militants in relatively small villages and towns. Al-Bab on the other hand is a city with a pre-war population of no fewer than 60,000 people – some estimates even put the population in the area as high as 100,000. The recapture of the border city of Jarablus in late August was a cakewalk for the Turks since ISIS clearly choose not to fight for a city on the periphery of its self-styled state. Al-Bab is 30 kilometers south of the Turkish border and ISIS have likely fortified their position there very well and are capable of bleeding out Turkey’s FSA proxies. A prolonged and bloody battle could be about to transpire.

The US has clearly thrown in its lot with the SDF, who have proven themselves a much more reliable and competent force against ISIS than the FSA forces. Manbij has a larger population than al-Bab (pre-war estimates put it at approximately 75,000) and the SDF were able to capture it using no less than 2,000 fighters with US-led coalition air support. That campaign took over two months but was still relatively decisive.

The SDF have also proven themselves as disciplined battle-hardened fighters, which is why Washington has chosen them to spearhead the Raqqa offensive, that they began earlier this month. The US will neither oppose Ankara in its fight against ISIS nor support its al-Bab offensive. The US has no interest in Turkey going on to fight the Kurds in Manbij after the al-Bab campaign, which is Turkey’s stated objective. A such move could ultimately compromise the SDF’s offensive against Raqqa, the crown jewel of the Syrian wing of ISIS’s terror state.

A long drawn out battle for al-Bab that could sap the Turkish-backed FSA factions manpower would be a blow to Turkey’s three-month-old Euphrates Shield operation and Ankara’s long sought goal to be a major power broker in the Syrian conflict in general.